Are my intentions clear for this alternative? - I don't want to use a separate stave. I rather hope the baritone will take the option and sing the high G, but I give the easier version as the standard. (Crotchet = 126.)

There may (or may not) be a snatched breath in the quaver rest, with the note before the rest only just there. The A# B and D are doubled by a horn starting mf then cresc. The baritone is expected to be ff throughout.

I found Finale's dashed curve better in this situation than the new(ish) dashed slur.

Seriously, perhaps you may be right, although the D was how I had it until I thought: ‘Why not go for a G?’ There is another high G without option (for now) about an hour later (again doubled by horn), close to the end of the piece (see screenshot - crotchet = 112). An E would fit the harmony on this later occasion, so maybe I should give it as an alternative. Any baritone who takes on this role, which is the dominant role in the two hour piece, is likely to be able to sing a high G as such. However, the role is substantial and demanding, and it requires effective and widely varied vocal acting &c &c, so one doesn't want to overdo things and risk straining the voice.

It's clear as is, but I would leave off "oppure", move the upper "jaws" so it is directly over the lower "jaws" (-: add extension lines to the upper "jaws" and "clay" and give the dashed slur more dashes by changing the setting so the dashes and spaces are smaller: it would be more visible.

Although I'd rather like it if the high G is sung in the first example as well as in the second (about an hour later) it is my original version of the first example that is notated with the full sized notes - the optional high G was an afterthought. In the second case the E should only be sung if the G is felt to be too risky at this stage of the piece.

What about the alignment of the semi-breves (whole notes)? They look slightly odd to me, but I suspect that this is correct.

That looks fine, David, and I think it gets your message across. I think that your alignment of the small whole notes with the large is standard. I couldn't find examples in Gould, but if they were on a separate staff, they would align like that. You might place the start of the small "jaws" more over the small note as you do "of".